Helium is a mono-atomic inert gas, and the smallest known gas molecule, and thus can pass through the smallest of openings. Therefore, helium is useful in leak detection. For example, helium gas may used to test the integrity of a vessel, such as an airtight or watertight container, by injecting the helium gas into the vessel and monitoring an outer surface of the vessel for the presence of helium atoms. Helium gas may likewise be used to test the integrity of other containers or conduits, such as gas pipelines, plumbing, fuel lines and the like.
Because of helium's chemical inertness, detection of helium is typically performed through identification of physical properties. For example, a mass spectrograph is able to identify the presence of helium by ionizing the helium atoms and then subjecting the resulting ions to magnetic and electric fields. This determines the relative mass of the helium ions and, through calibration against an alternate known gas, determines the presence of helium. However, detection of helium using a mass spectrograph or other ionization or infrared spectral tool, requires large and expensive equipment, and typically consumes large amounts of power.